MRI machines produce magnetic force measured in tesla (T), a unit of magnetic flux. Most MRI machines range between 1.0 T to 1.5 T, with more powerful units at 3.0 T. There are two features of a magnetic field that are the source of most MRI incidents: (1) projectile (missile effect) and (2) translational attraction. The projectile effect causes ferrous-based materials, nickel alloys and most stainless to be pulled violently toward the magnetic source. Translational attraction occurs when one point of an object in a magnetic field is attracted to a greater extent than the object's furthest point from the attracting source. This differential creates a more forceful attraction, increasing the speed with which the object may move toward the magnetic source.
These effects make all metal containing objects highly dangerous in the MRI environment. For example A hairpin or paper clip within the 1-3 T line range could reach a velocity of 40 mph and will be attracted to the center of the generated field (x, y and z axis) where the lines of force are equal.
All pacemakers and implantable cardioverter/defibrillators should be considered contraindicated under any circumstance in the MRI room. When these devices are exposed to an MRI environment, a life-threatening condition may be created within the 3 T line. Intravascular catheters, intubation equipment, infusion pumps, orthopedic implants, stents and other devices should be verified as MRI-safe in the prescreening.
All patients, visitors, parents/guardians of patients and MRI hospital personnel must pass through an MRI screening process for entering the MRI room. To date, the screening process includes questioning of the subject about to enter the MRI room. Questioning is an important source of information but not always reliable. This method depend on communication skills and memory of the subject that are not always adequate to ensure safety. In addition to that there is the possibility that responsible personnel forget to question a patient about metals attached to him.
Thus there remains a long felt need for a reliable objective method for screening subjects prior their entrance to the MRI room that are not based on communicating with the subject. In addition, this method should be carried out in parallel to another action so that it will not be dependent on the memory of a staff person.